1. Field of the Invention
This invention relates to isolators usable in the optical wavelength region and more particularly to optical isolators suitable for optical fiber communications and the like.
2 Description of the Prior Art
Recently, as research and development in the field of optical fiber communications advance, the communication technique is finding increasing practical application and, with improvements made in performance, including reduction in insertion loss, of various optical devices used in optical fiber communications, such as optical fibers and coupling circuits between a light source and optical fibers, a number of problems have been given rise to as described below: First, owing to the fact that the light reflected as at the end face of an optical device is allowed to return to the light source without any substantial loss, the operating characteristic of the light source is adversely affected particularly in cases where the light source takes the form of a semiconductor or other laser oscillator and, in some cases, there arises the problem of communication quality being deteriorated to an extreme extent. Another problem recently arising is that of multiple reflection between the end faces of optical devices that results in appearance of echoes in the signal being transmitted.
One of the measures taken to solve these problems is the use of an optical isolator that utilizes the Faraday effect obtainable in a material subjected to a magnetic field. The basic structure of such optical isolator includes a Faraday rotator effective to rotate the plane of polarization of the incident light beam by approximately 45 degrees and a pair of polarizers, i.e., optical elements serving the function of extracting the linearly polarized component from the light passing through the elements, which are arranged on the opposite sides of the Faraday rotator with their azimuthal angles making 45 degrees relative to each other. Such optical isolators usable in the 0.8 .mu.m region and those usable in the 1.3 .mu.m region have been proposed in the article entitled "Optical Isolators for Optical Fiber Communications" (in Japanese), by Seki, Kobayashi and Ueki, Technical Reports OQE 78-133 of the Institute of Electronics and Communication Engineers of Japan. According to this proposal, polarizers in the form of Rockon prisms are employed and only limited reduction in cost is obtainable since generally fabrication of polarizing prisms such as Rockon prisms inevitably includes the step of cementing together the prism elements previously precisely polished, and which makes any substantial reduction in fabrication cost of such prisms considerably difficult.
Accordingly, it is highly desirable from the standpoint of cost reduction to construct an optical isolator without use of any components necessitating the precise polishing process such as polarizing prisms. The simplest method suited to this end is to employ calcite or other birefringent crystal plates as polarizing elements, as proposed in the Japanese Patent Public Disclosure Number 53-149046 entitled "Optical Isolator," which corresponds to the U.S. Pat. No. 4,178,073. The optical isolator proposed therein includes a first rod lens in the form of a light-focusing transmission body of a quarter pitch length in the direction of light travel, a first birefringent crystal plate of calcite, a Faraday rotator effective to rotate the plane of polarization by 45 degrees, an optically active crystal element of quartz for 45.degree. rotation of polarization, a second birefringent crystal plate of calcite, and a second rod lens, all arranged in that order. With this optical isolator, the incident light beam, emerging from a first optical fiber arranged on the entrance side of the isolator, is coupled to a second optical fiber on the exit side thereof without any substantial loss, and any light beam returning from the second optical fiber, leaves the first birefringent crystal plate, following therein a path distinct from that of the incident light beam on account of the nonreciprocality of the Faraday rotator. Superimposition on the incident light of the reflected light at the entrance end of the optical isolator is prevented by arranging so that the reflected light leaves the first birefringent crystal plate at such a point thereon as to enable it to proceed clear of the first rod lens or by arranging a light stop in an appropriate position to intercept that portion of the reflected light which may otherwise enter the first rod lens.
Such structure does not involve any particular problem as far as the functioning of the device is concerned but the two birefringent crystal plates employed therein are required to have a considerable length in order to ensure that the reflected light does not strike on any portion of the cross section of the first rod lens or to ensure that only a limited portion of the reflected light is allowed to enter the first rod lens. For example, where the rod lens has a diameter of 2 mm, the birefringent crystal plate should have a length of about 20 mm, in order to enable the reflected light to pass completely clear of the rod lens. This makes it impossible to reduce the cost of such isolator elements to any satisfactory extent.
Further, in certain optical isolators, only a single conventional spherical lens has been employed to enable reduction in length of the birefringent crystal plates. In this case, however, the distance between the conventional spherical lens and the laser oscillator or the adjacent optical fiber must necessarily be increased, making it difficult to reduce the insertion loss of the optical isolator for improved coupling efficiency.